The present invention relates generally to integrated circuit assemblies and more specifically to thermal management for integrated circuit assemblies.
Integrated circuit die directly mounted on flexible substrates are referred to as flip-chip-on-flex (FCOF) (or flexible circuit) assemblies. FIG. 1 shows a simplified cut-away drawing of an example of a conventional FCOF assembly 100. The FCOF assembly 100 includes a flexible substrate 110 on which is mounted a flip-chip die 101. Multiple die pads 115 are disposed on a surface of the die 101 facing the substrate 110 (Note: FIG. 1 shows only two of the die pads, in cross-sectional view).
The flexible substrate 110 is composed of polymer film layers 111(a) and 111(b) bonded to a metal trace layer 112 with adhesive layers 113(a) and 113(b), respectively. The flexible substrate 110 includes multiple conductive pads 116 that are formed where the metal trace layer 112 is exposed through the polymer film layer 111(a) and the adhesive layer 113(a) (Note: FIG. 1 shows only two of the conductive pads, in cross-sectional view). The multiple conductive pads 116 are positioned such that they coincide with the multiple die pads 115 when the die 101 is mounted on the substrate 110. Multiple solder bumps 119 are used to bond and electrically connect the corresponding die pads 115 and conductive pads 116.
An adhesive underfill layer 118, typically made of an epoxy material, fills the space between the die 101 and the flexible substrate 110. The underfill layer 118 is used to support the die 101 mechanically while controlling the stresses that may develop in the solder bumps 119.
A stiffener 125 is bonded to a surface of the flexible substrate 110 opposite that of the die 101 using an adhesive layer 113(c). The stiffener 125, typically composed an aluminum sheet, is primarily used to stiffen the flexible substrate 110 so that it provides a planar surface for mounting the die 101.
Conventional thermal management techniques for FCOF and other types of circuit assemblies such as the use of heat sinks are becoming increasingly inadequate due to several factors. First, integrated circuits are being operated at increasingly high frequencies, thereby generating more heat. At the same time, integrated circuits and circuit assemblies are increasing in density, causing the heat to be concentrated in a smaller area. As a result, the temperature of integrated circuits in these assemblies is increasing to the point of adversely affecting their reliability.
Accordingly, there is a need for a more effective way to dissipate heat generated by integrated circuits in FCOF and other types of circuit assemblies.